DC-DC converters are commonly used to supply DC power to a variety of electronic systems and devices, such as, but not limited to, relatively low voltage circuits, such as personal computers and portable digital assistants, as well as high voltage integrated circuits (e.g., automotive electronic subsystems) and the like, and are available in a variety of configurations for deriving a desired DC output voltage from a given source of DC input voltage. As a non-limiting example, the DC-DC converter may be configured as a buck mode architecture, including one or more power switches, current flow paths through which are coupled between a DC input voltage terminal and a reference voltage terminal (e.g., ground), and the common or phase node between which is connected through an output inductor to an output voltage node, to which a storage capacitor and the powered load/device are connected. By controllably switching the power switches on and off, the upstream end of the output inductor is alternately connected between the DC input voltage and the reference voltage. This produces an alternately ramped-up and ramped-down output current through the inductor to the output node, so as to deliver a prescribed DC output voltage to the load.
A voltage mode DC-DC converter, which is typically used in applications where the load current demand is relatively large, includes a voltage control loop having an error amplifier, the output of which is used to control a PWM comparator, which generates a PWM voltage waveform. This PWM voltage waveform is applied to driver circuitry, which controls the turn on/off times of the power switches, in accordance with transitions in respective PWM voltage waveforms with which it drives the power switches. To meet the demand for substantial load current, the PWM waveforms that control the on/off switching of the power switches are typically mutually complementary, so that a conductive path from one or the other of the input voltage source and ground will be continuously provided through one or the other power switch to the output inductor. This mode of operation is customarily referred to as continuous conduction mode (CCM).
To regulate the DC output voltage, a voltage representative thereof is fed back to the error amplifier and compared with a DC reference voltage to produce an error voltage. This error voltage is amplified and filtered to produce an input signal to a PWM comparator, and is compared thereby with a sawtooth voltage waveform, to produce the PWM waveform, the pulse width of which is defined in accordance with the crossings of the error voltage threshold by the sawtooth voltage waveform.
One of the main issues faced by the PWM controller is the need to provide over-current protection; for this purpose, the PWM controller employs an over-current detection comparator. For low voltage applications (e.g., on the order of one to only several volts), such as in personal computers and hand-held devices, this comparator has been customarily implemented using low voltage components, such as bipolar or metal-oxide-semiconductor (MOS) transistors, whose manufacturing tolerances can be readily replicated, and thereby ensure the production of well matched and parameter-predictable devices that will enable the comparator circuitry to accurately sense variations in its monitored inputs.
However, such low voltage device-based architectures cannot be used in relatively high voltage applications (e.g., those employing voltages on the order of three to twenty-eight volts or greater), since such relatively large voltages would damage or destroy the low voltage devices. Although high voltage devices, such as double diffused MOS (DMOS) transistors, are able to handle these increased voltage levels, that are not suitable for implementing precision or high electrical sensitivity input stages that are required for amplifiers and comparators, such as those employed in an over-current detection comparator. Their use is presently limited to a relatively small number of circuit functions, such as switches, cascade devices, high voltage application devices, and DC-biasing devices. Consequently, there is a need for a comparator that is able to handle high input voltages, while being able to monitor relatively small variations in such voltages with the sensitivity and precision that is currently available only in low voltage implementations.